Borrelia burgdorferi, the causative agent of Lyme disease, establishes persistent infection that can affect the joints, heart, skin, and nervous system. The abilities of this spirochete to disseminate from the site of inoculation, a tick bite, and to persist despite the host immune response, indicate that interactions with mammalian cells occur continually during infection. We showed that B. burgdorferi binds to integrins, which are cell-surface signaling receptors key to many functions, including maintaining tissue integrity and the immune response. The B. burgdorferi outer membrane protein, P66, is a ligand for the [unreadable]-3-chain integrins. B. burgdorferi mutants that do not express P66 have an ID50 of at least six logs over that of the parental strain. The mutants are unable to establish infection even when introduced by infected ticks. P66 also affects gene expression in mammalian cells. Functional pathways in both epithelial and endothelial cells that were affected by the presence vs. absence of P66 included focal adhesion and regulation of actin cytoskeleton. These pathways participate in host responses to pathogens such as phagocytosis and the oxidative burst, as well as in maintaining cell shape and tissue integrity. Further work has suggested that P66 causes disruption of actin structures within human cells in culture. The VEGF pathway was also affected in endothelial cells by the presence of P66, and the presence of B. burgdorferi has effects on endothelial cells that mimic some of those of VEGF. VEGF promotes vessel permeability, and this activity may facilitate B. burgdorferi dissemination from the site of inoculation by the tick, and might promote tick feeding efficiency. In fact, ticks infected with B. burgdorferi fed to larger repletion weights than did uninfected ticks. The p66- mutants are not culturable even from the inoculation site within a few days, suggesting that they are rapidly cleared. Based on these results, we hypothesize that P66 is required for the bacteria to adapt to and survive within the mammalian host, or that P66 affects the host response to the bacteria such that it is unable to clear wild-type B. burgdorferi. In Aim 1, we will test the hypothesis that P66 is required for B. burgdorferi transition to and or adaptation to the mammalian host environment. In Aim 2, we will test the hypothesis that P66 is required for B. burgdorferi to survive the host response to the organism. In each Aim we will test our hypotheses in the mouse model of infection as well as in a variety of in vitro approaches. Our overall goal is to understand how P66 allows B. burgdorferi to overcome host barriers to the establishment of persistent, disseminated infection. As Lyme disease prevalence continues to expand in the Northern hemisphere, in terms of both case numbers and geographic distribution, this work may illuminate not only how B. burgdorferi causes Lyme disease in humans, but also how the integrin ligand P66 affects the biology of B. burgdorferi at the bacterium's interface with the mammalian host and tick vector.